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M923A2 - Loss of Power While Driving

crolfe1984

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Excellent tips Dan! Addressing this step by step starting with a new pickup tube. Have to drop the tank to do it correctly, so I'm having it towed to a local shop Monday.

On another note, I DID FIND a source for the brass elbow that connects the pickup tube to the steel fuel line. New one on the left, old on the right. See pics below...

IMG_5352.JPGIMG_5353.JPG

I got them from Tipco Technologies Inc. (a local commercial hose & fittings supplier). They went for $8.50 per and are item number AER 402X8, 1/2 x 3/8 BR Male Elbow. They are an EXACT fit.
 
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TEAMJENSEN

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I'm a new 5 ton owner and was driving down a flat grade my truck lost power, engine was running fine but rpms dropped and truck started to down shift even as I gave it more gas. I looked at my m923a2 and it does not have the small in line fuel filter. I did change the spin on fuel water separator filter and the canister filter under the driver side wheel well. I took it out again with the same result. I waited a couple days filled the fuel tank and she ran fine for 1 day about 30 miles. Today it did the same thing in the first mile. According to the feed I should start at the tank? What am I exactly looking for, again this is all new to me. Thank you in advance. I have really learned a lot from you guys and appreciate all the help. I have already wired the reverse lights using the transmission switch mentioned in a different thread. Again. I'm so glad to have found such helpful and knowledgeable friends.

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dangood

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Start easiest first.
Make sure tank has fuel. And more than 10 gallons as I have ran out multiple times, and after driving hundreds of these trucks and running out at quarter tank...know it is more common than not, (Wish I didn’t have to say that)

If no go, Check fuel solenoid, (driver side of engine, above the fuel lever in side of injector pump) make sure it sucks all the way in and STAYS sucked in after starting the engine, and does not slowly push back out, hitting the fuel lever into the closed position. A sure sign of a bad one is it slowly moving, or getting HOT quickly. Also check the manually operated kill lever on bottom of dash just to the right under steering wheel, as it has a bicycle cable that pulls the fuel lever to the off position when pulled from inside the cab, and when pushed back in can often just kink up the cable under the hood instead of pushing the lever, or not quite fully open the fuel lever which could limit the fuel flow.

If no go, Next is push primer bottom on lift pump, (driver side engine, lower middle area of engine) and check for prime. If button pushes easily, and does not get harder after a few dozen pushes, you have lost prime and not in a easy couple minute fix.

If you can’t get solid prime, Next requires tools, as filters and water separators are next, and make sure to fill them up if possible with clean fuel, and re-prime using the lift pump primer button.

If no go, Next I would pressurize the fuel tank with a fuel cap with a valve stem installed, or a grocery bag with some rags or something soft inside and a blow gun with the rag-bag pushed as tight as possible around the blow gun tip inside the fuel inlet and try to create a decent seal for 20-30 seconds (3-4 psi is PLENTY, or for sure stop if your tank expands with a loud boink sound. This is much easier with two people as you want to inspect for leaks in the fuel lines while it’s under pressure. So many times it is the fuel lines under the cab are cracked, but not very visible until you see dripping or a wet spot start to appear on the hose, or around a hose clamp connection. Fairly easy to replace though.

If you still can’t get consistent prime, Next I would open the bleed screw on the injector line right above the water separator filter (driver side, up high above the lift pump, and to the rear of engine a bit), and allow the pressured tank to push any air out, as you also have someone push the primer button like a madman, then close the bleeder, and finish off priming with the lift pump button until hard, or you hear pressure building inside of injector pump. (It hisses slightly when fuel starts to flow into it after it has lost prime)

If no go still, next I would remove the fuel line from the top of the tank and pull out the copper pickup tube and inspect for hairline cracks near the top of the copper tube just under the treads that screw into the top of the tank. (Have delt with this many times, and is the main cause of phantom losses of prime when everything else is fine)
Next is really inspect all the injector lines, fuel lines, filters, etc again with pressureized fuel tank again, with a clear chemical grade temporary clear fuel line to visibly watch fuel flow, and to witness any bubbles, and where they come from. This is where lift pump issues will reveal themselves, or any air bubble entrances into the fuel system can be seen.
If you don’t fix it by now, it may be Injector Pump itself, which is a whole new level of **** it.
 

WillWagner

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Or an overfolw valve. A failed oveflow valve will not let the galley pressure get the the pressure needed to make correct low pressure pressure. If you don't get that, the correct high pressure can't be made.
 

Jbulach

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Or an overfolw valve. A failed oveflow valve will not let the galley pressure get the the pressure needed to make correct low pressure pressure. If you don't get that, the correct high pressure can't be made.
Hey Will, any idea what the minimum gallery pressure is?

I lost prime after sitting for about a month. Now I seem to be slightly lacking power, only at WOT, and also lost about 5 mph off top speed empty on the highway. I have not had any more loss of prime, runs smooth, fuels clear and clean, tanks 3/4 full, filters have very few gallons on them, all rubber lines look brand new, as well as the tank pickup tube. I did replace the return line dip tube that I sheared of at Haspin, and I had been running without it when I lost prime. I have ran 5 or so, 60 mile trips to get gravel since with no improvement in power.

My primer button does leak fuel a little only when pumping, so maybe sucking air also?

So before swapping lift pumps and or overflow vales off my parts motor, I installed a pressure gaugeIMG_5921.jpgIMG_5922.jpg, reprimed the pump fired right up, only registered about 4psi at 450rpm for about a minute (maybe due to air in the new gauge and line?) then climbed to 18psi and holds steady from idle to no load WOT. IMG_5925.jpgI haven’t loaded it yet since I’ve added the gauge, but thought I read somewhere that 28-32psi was normal?
 

Jbulach

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Running more like 10-15psi on the road. Guess I’ll start by switching out the overflow valve and see what I get.
 

WillWagner

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25-35 is normal, most new valves fall in around 30. It is somewhat normal to see the needle on the installed gauge to bounce around, like 15 to 30 at idle but the pressure should stay fairly steady when loaded.

Restriction will cause low fuel pressure too. IIRC, 4 in suction restriction and a 10 psi drop across the pressure filter....yes, three places to check, suction into lift pump from the tanks, pressure into the pressure filter and pressure out of the pressure filter. that's why it is a good thing to have multiple test fittings and gauges. interrupt the system once and find the issue then.
 
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Jbulach

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Well that didn’t give me the results I was looking for... I swapped my in a used overflow valve of unknown condition, went from 17psi down to 7, put the old one back in, back to 17psi, ordered up another set of new filters before I mess with anything else.
 

74M35A2

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Cummins 6CTA8.3 service manual states mechanical lift pump should be pushing minimum 20psi of fuel pressure at high idle. Mechanical pump volume flow test with open hose into container is 150ml (5oz) at 30 seconds of cranking with fully charged batteries and minimum 120rpm crank speed.

I don't like to see starters ran that long, and would simply cut the volume flow in half @ half the time. Or, just slam a new pump on it and go for a ride. Done. They don't last forever. I replaced mine, just to be ready to eat Simp's lunch.



Test 1: Output Pressure Test (Piston Style)
Operate the engine and measure the output pressure of the fuel lift pump using an in-line pressure gauge at the inlet to the injection pump.
Minimum pressure at high idle is 138 kPa [20 psi].










Disconnect the fuel shutdown solenoid wire.
Measure the engine cranking speed with a handheld tachometer, Part Number 3377462.
The minimum cranking speed is 120 rpm.


LARGE



CAUTION Leave the shutdown solenoid disconnected for the following check:

Disconnect the output pressure line from the fuel lift pump and run it into a container.
Crank the engine for 30 seconds and measure the fuel lift pump flow volume.
The minimum flow volume is 150 mL [5 oz].




 
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WillWagner

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Maybe air, possible the 2nd valve was way worse than the original one. Air will usually cause a slow/hard start to start issue, low fuel in the galley, same as a valve that has a completely broken spring or the ball held open, that lets the fuel drain from the galley, a failing/low pressure valve will let the engine start fine but have low power/RPM, galley full, not draining back or flowing out the return.
 

Jbulach

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Replaced the primary canister filter today, it did have quite a bit of junk in the canister but the element looked pretty good. No change in pressure, secondary spin on pressure filter will be here tomorrow, not expecting much. Will probably try feeding the lift pump from a bucket for the heck of it, then swap out the lift pump next.
 

Coffey1

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I had the rubber connector come loose on the intercooler to turbo and it run like you are saying.
 

Jbulach

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Yeh, I should probably check that side to, but the loss of prime and low fuel pressure lured me toward looking at the fuel.

Last night I had the boy watch my gauge while I pumped the primer and was getting 25PSI, so I’m really suspecting the lift pump, plus maybe the overflow valve for a few more PSI. Will likely put an adjustable valve on once I get a clearer picture of whats up.

I have a pyro setting on the tool box that I need to get installed, so I can get a baseline before I start tweaking on the pump, probably oughta order a boost gauge as well, considering how much I like data, for what I mess with.
 

Jbulach

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So, I got both filters changed with no improvement.

Checked the turbo hoses, found no leaks but look like replacement needs to be put on the to do list.

Swapped out the lift pump with the “newer looking” Carter from my parts truck,IMG_6048.jpgno change in pressure, but the pressure is much slower to bleed off after shut down. This leads me to believe that my one time loss of prime, was due to a faulty check in the old pump, in combination with no dip tube on my tank return line, and sitting for an extended period.

So, at this point I’m thinking my new gauge is junk, IMG_6051.jpgbut verified to be within 1/2 psi of my OTC test gauge.

Switched my parts overflow valve back in one more time, back down to 7psi, original back in, back up to 17psi. Shut the truck off and could get 22psi pumping the primer as fast as I could.

At this point I’m thinking spend the $50 and buy the new adjustable overflow valve I’ve been wanting anyhow.

I’ll probably do a flow test of the lift pump, out my gauge port, at the input side of the pressure filter, as well as running the entire fuel system, including the pickup tube up to around 15psi to check for leaks, and verify theres no trash in the tank.

I wonder if theres any chance I’m loosing the lobe off the cam that runs the lift pump? The face of the piston that rides on it looks fine, but I did’t think to look in the block while I had the pump off.
 
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162tcat

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I didn't read the whole thread but any chance your supply line is collapsing and cutting off the fuel flow while you drive?


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Jbulach

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Anythings possible, lines “look” new, and there are other clues that they have been replaced fairly recent. If it gets to checking for that I will likely do as Will suggested and check how much suction is on the inlet side of the lift pump, before I replace them.
 

Bill Nutting

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My M934 was loosing prime. Got some advice to install an electric fuel pump at the tank. Bought a 24 volt pump, installed it and found a pin hole leak in a line in about 10 minuets. I repaired the line and stopped loosing prime. The electric pump cost me about 5 MPH so I removed it but left all the fittings in place. This gives me a quick way to pressure test the fuel system if I have issues in the future. I carry that pump with me for emergency’s. I could reinstall it in about 20 minuets. I think these trucks should have electric pumps in the tank. It would provide more reliability and it would make finding leaks in the fuel system a snap.
 
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